Various communication systems have been proposed for remotely monitoring and controlling the use of electric energy supplied to large numbers of customer locations. Large numbers of geographically disbursed residential electric power customers are supplied electric energy by distribution networks. It is often desired to control and monitor the levels of electric energy supplied by the distribution network from a central control station. One system for a remotely controlled or automated distribution system is disclosed in U.S. Pat. No. 3,911,415, wherein bidirectional carrier communication signals are transmitted over power line conductors between a central station and each of large numbers of electric power customers. U.S. Pat. No. 3,659,280 discloses a power line carrier communication system wherein plural transmitters and receivers are operated at different selective frequencies of signals transmitted over the power lines. U.S. Pat. No. 3,980,954 discloses and claims a bidirectional communication system which utilizes a VHF-FM broadcast transmitter signal which is modulated with interrogation or command signals transmitted to radio receivers at each customer location. Response signals from the customer locations are transmitted by carrier signals applied through the power lines.
In U.S. Pat. No. 3,815,119, a remote meter reading system as described utilizing electric power lines and a mobile radio interrogating arrangement. A plurality of meters have separate transmitters for transmitting meter data at different frequencies to receivers mounted on utility poles and connected to the power lines to receive the transmitter signals. The receiver signals are coupled to a transponder which includes a radio transmitter. Transmitted radio signals are produced including serial transmissions of the meter readings of each separate meter.
In U.S. Pat. No. 3,919,640, a remote meter reading system includes an RF radio communication link. Digital and analog data signals are transmitted on different frequency channels through the radio link. In U.S. Pat. No. 3,656,112 a remote utility meter reading system includes a communication system which produces carrier signals over the power line and also a radio link. A general addressing technique is utilized in each of the aforementioned systems wherein each remote location includes a unique binary coded address and the communication signals are transmitted in a serial fashion with each remote location being signaled or responding in a sequential manner with the associated location being designated by a unique code.
The term "power line carrier" (PLC) is commonly used when referring to the art of transmitting communications or controls signal over a power line. PLC was initially utilized by power utilities as a means for controlling widely distributed loads. Since its inception, PLC has developed and been used to control circuit breakers, synchronized clocks, and transmit voice and computer data. However, despite the wide variety of existing uses, PLC has never become a popular communications medium.
Power lines are a hostile environment for communications signals. PLC transmitters need to be powerful to overcome electrical noise and signal attenuation caused by all of the equipment on the power line. Because of this, most PLC has a very limited range, e.g. within the perimeter of a house. It requires expensive and powerful commercial PLC systems to signal from a local substation to a customer's house. Most modern PLC systems send only data.
In modern PLC systems, the system architecture is such that it is advantageous to send data as fast as possible. This requires the PLC communication equipment to have as wide a bandwidth as practical. The need for fast response and the traditional difficulty of making narrow bandwidth filters has effectively prevented engineers from seriously considering ultra narrow bandwidths for PLC systems.
It has long been understood among communication engineers that as the bandwidth of a communications system is narrowed (thereby slowing it down), it becomes more immune to noise. Until now, due to the practical considerations discussed above, power line carriers have not been able to carry this principal to extremes.
It is therefore a general object of the present invention to provide an ultra narrow bandwidth data transmission system for use on electric power distribution systems for achieving superior signal to noise ratio under a given transmission power.
Another object of the present invention is to provide an ultra narrow bandwidth data transmission system to a power line carrier, to thereby achieve high signal reliability using very low transmitter power.
Another object is to provide an ultra narrow bandwidth data transmission system capable of operating over power transmission lines to provide pertinent data regarding the power transmission lines.
These and other objects will be apparent to those skilled in the art.